1. Field of the Invention
This invention generally relates to an access control device and more particularly to a servo circuit of an optical disk (or disc) drive unit of an optical disk system (namely, a magneto-optical disk system). Especially, this invention relates to a servo circuit suitable for an optical disk system employing what is called a 1-beam push-pull track error detection method.
2. Description of the Related Art
First, a conventional optical disk system will be outlined hereinbelow.
The conventional optical disk system has an optical pickup device (namely, an optical head) used to converge laser light emitted from a semiconductor laser and to irradiate the laser light onto a target position on a disk medium to thereby recording and reproducing information. Such an optical pickup unit is comprised of an optical system and a driving system.
The optical system is a mechanism for converging laser light onto the disk medium and detecting the difference between the actual position of a laser light spot and the desired position (namely, the target position) on the disk medium. Further, the optical system consists of a semiconductor laser, lenses, a beam splitter, a photodiode or the like.
The driving system has a driving mechanism for maintaining the fixed positional relation between the actual position of the laser light spot (namely, the laser beam spot) and the target position on the disk medium by performing a focusing control operation of making an objective lens follow an axial deflection of an optical disk, as well as a tracking control operation of making the objective lens follow a radial deflection. Further, the driving system is mainly comprised of a magnet, a coil and a supporting member.
Additionally, the driving system has a further driving mechanism for driving a seeking control system to move the entire optical-pickup device to a target position in the direction of a radius of the optical disk.
The conventional optical disk system further has an access control device which is a control device for moving a laser beam spot to a target position.
Hereinafter, the access control device of the conventional optical disk system will be outlined by referring to FIGS. 1(a) and 1(b).
FIG. 1(a) schematically illustrates the configuration of a first example of the conventional access control device.
In the device of FIG. 1(a), a slider 18 is adapted to move in the direction of a radius of an optical disk 10 by being guided by a rail 19. Further, the slider 18 is driven by a coarse-motion motor (hereunder referred to simply as a coarse motor) as 21 driven by a slider drive circuit 9.
Generally, an operation of the optical disk device, by which the optical pickup device moves (or is moved) onto a desired sector of a target track, is referred to as an access operation thereof. Moreover, a part of this access operation, by which the optical pickup device moves in the direction of a radius of the optical disk to the vicinity of the target track, is referred to as a seeking operation.
Furthermore, in case where the distance to be made by the optical pickup device is large, the seeking operation is referred to as a coarse seeking operation. In contrast, in case where the distance to be made by the optical pickup device corresponds to one track to hundreds of tracks, the seeking operation is referred to as a fine seeking operation.
As shown in FIG. 1(a), the whole optical pickup device 2 is fixed to the slider 18. The coarse seeking operation of the optical pickup device is conducted according to an instruction issued by a microprocessor unit (hereunder abbreviated as an MPU) 12. Thereby, the slider 18 is moved by operating the coarse motor 21 so as to move the laser beam spot to the vicinity of the target track.
In contrast, the fine seeking operation is an operation of moving the laser beam to a track, which is distant from the current position thereof one to hundreds of tracks, by stopping the slider 18 and effecting what is called a track jumping.
As shown in FIG. 1(a), an objective lens 3L is connected to a tracking actuator 3 through a spring member and is adapted to move under the control of the tracking actuator 3.
The laser beam first passes through the objective lens 3L and is then reflected on a recording surface of the optical disk 10. Subsequently, the reflected beam is incident upon a photodiode 23 for detecting a signal through a beam splitter (not shown) and a condenser lens (or a converging lens (not shown)). The photodiode 23 is a quadrant photodiode, and the difference in position (in the direction of a radius of the optical disk) between the current position of the laser beam spot and the target track is detected by a track error detection circuit 4 by effecting what is called a 1-beam push-pull method and using a plurality of outputs of this photodiode as represented by a tracking error signal TE.
A tracking control circuit 13 and a track jumping control circuit 13A are controlled according to the signal TE outputted by the track error detection circuit 4.
Further, during the optical pickup device 2 moves above a spiral track a slider normal-control circuit (hereunder sometimes referred to as a slider control circuit) 14 controls the slider drive circuit 9 to move the slider 18.
The position of the objective lens 3L is detected by a lens position sensor 6. Further, a signal representing the detected position of the objective lens 3L is inputted to both of the slider control circuit 14, which controls the slider drive circuit 9 to move the slider 18 in such a manner that the sensor 6 outputs no signal representing the detected position of the objective lens 3L (namely, the center (or optical axis) of the objective lens 3L corresponds exactly to the optical axis of the laser beam) at the time of recording or reproducing information to be stored in the optical disk, and a lens holding circuit 7A (shown in FIG. 4(a)) which holds the objective lens 3L at a predetermined position. Further, outputs of the tracking control circuit 13, a brake circuit 13B (to be described later) and the track jumping control circuit 13A are connected to input terminals 62, 64 and 63 of a switch 15A, respectively. A tracking drive circuit 5 for driving the tracking actuator 3 is controlled in accordance with a signal outputted from an output terminal 61 of this switch 15A.
The position of the slider 18 is detected by a linear encoder 20, the output of which is applied to a slider position control circuit 17 for stopping the slider, to a slider speed control circuit 8 for controlling a speed at which the slider is transferred, and to the slider normal control circuit 14. Output terminals of the circuits 17, 8 and 14 are connected to input terminals 66, 67 and 68 of a switch 16A, respectively. The slider drive circuit 9 for driving the coarse motor 21 is controlled according to a signal issued from an output terminal 65 of the switch 16A.
Further, the connecting relation between the switches 15A and 16A is controlled by the MPU 12. For example, in case of performing a coarse seeking operation, the terminals 65 and 67 are connected to each other. Thus the slider 18 is transferred at a high speed. When the slider reaches a desired position, the terminal 65 becomes connected to the terminal 66 instead of the terminal 67. As a result, the slider 18 is quickly stopped and the terminal 61 is connected to the terminal 64.
In case of effecting a fine seeking operation, the terminals 65 and 61 are connected to those 68 and 63, respectively. Furthermore, in case of controlling a tracking operation, the terminals 65 and 61 are connected to the terminals 68 and 62, respectively.
Meanwhile, the access operation of this conventional optical disk system will be described hereinbelow by referring to FIG. 1(b).
First, in steps S1 and S2, the number of revolutions per predetermined time (namely, the rotation speed represented in terms of, for instance, revolutions per minute (rpm)) of a rotation system including the optical disk is set to be a desired number of rotations thereof.
Next, in step S3, the system starts an operation (namely, what is called a focussing servo operation) of controlling the position of the objective lens 3L to set the waist of the laser beam on the recording surface of the optical disk.
Subsequently, in step S4, it is confirmed that the focussing servo is locked.
Then, in step S5, a track pull-in operation (to be described later), which is preparatory for a tracking operation, is performed.
Next, in step S6, a tracking servo circuit becomes on, namely, a tracking operation is effected. If it is found in step S7 that a good result of the tracking operation is not obtained, the tracking servo circuit becomes off in step S8 and the track pull-in operation is repeatedly performed.
Thereafter, the information indicating such an address (hereunder sometimes referred to as the address information) is read in step S9. If it is found in step S10 that the distance between the read address and the desired or target address (namely, the absolute value of the difference between the current track corresponding to the read address and the target track corresponding to the target address) is equal to or greater than a predetermined value (n), a coarse seeking operation is carried out (namely, the laser beam is moved according to such a distance) in step S11. This coarse seeking operation is effected by the slider speed control circuit 8 and the slider position control circuit 17. In contrast, if it is found in step S10 that such a distance is less than the predetermined value (n), the system judges in step S12 whether or not the current track matches the target track (namely, whether or not the difference between the current and target tracks is equal to 0).
Meanwhile, if the coarse seeking operation is performed in step S11, the track pull-in operation and the tracking operation (hereunder, these operations will be sometimes referred to as the tracking control operation collectively) are performed again in steps S5 and S6 under the control of the tracking control circuit 13 from a position of the laser beam, which is set by the seeking control system 8 and 17, to read the address information.
In contrast, if it is found in step S12 that the address illuminated by the laser beam does not matches the target address (namely, the current track does not match the target track), the system performs the following operation (namely, what is called a track jumping operation) in step S13 to make the laser beam reach the target address. Namely, the laser beam is made reach the target address not by being moved one track at a time but by being continuously moved at a time in response to the difference between the current track and the target track. This operation mode (namely, the mode of the track jumping operation) is controlled by the track jumping control circuit 13A.
However, a conventional optical disk drive for driving an eccentric optical disk (for example, a compact disk (CD)) has a drawback that in case where a radial deflection due to the eccentricity is large, the tracking control operation is commenced and performed stably in steps S5 to S8.
FIG. 2 is a waveform chart for illustrating the waveform of an example of the tracking error signal.
As is seen from FIG. 2, the tracking is not started if the tracking control circuit becomes on when the relative speed (hereunder sometimes referred to simply as a radial relative speed) of a track in the direction of a radius thereof is large (namely, the frequency of the tracking error signal TE is high but the period thereof is small).
This is due to the fact that the laser beam needs to be rapidly accelerated in the direction of a radius of the track because the laser beam, which has been stopped, should follow the track which is moving in the direction of the radius thereof.
There have been developed conventional access control devices to eliminate the above described drawback of the conventional optical disk drive.
FIG. 1(a) illustrates a first example of such a conventional access control device (hereunder sometimes referred to as a first conventional access control device). Further, the access control device of FIG. 1(a) is characterized by employing the brake circuit 13B. This brake circuit 13B requires an RF signal, the phase of which is shifted by 90 degrees from that of the tracking error signal TE. However, in an unused region of a write-once compact disk read-only memory (WO CD-ROM), there is formed a guiding groove, the depth of which is nearly half of that of each pit of an ordinary CD-ROM, by being slightly wobbled by marks (or pits) corresponding to address codes. Namely, no pits are formed in the unused region of the WO CD-ROM. Therefore, no RF signals can be obtained from such an unused region of the WO CD-ROM. Thus mirror signals cannot be stably generated. Consequently, the brake circuit cannot be used.
Next, a second example of the conventional access control device (hereunder referred to as a second conventional access control device) will be described hereinbelow. Note that the second conventional access control device does not employ a special circuit such as the brake circuit of the first conventional access control device.
FIG. 3 illustrates the waveform of a tracking error signal of the second conventional access control device. As shown in this figure, it is detected at a moment t1 that the relative speed of a track with respect to a laser beam becomes equal to or less than a predetermined speed (namely, the period T1 becomes equal to or longer than a predetermined period of time). At that time, the tracking control circuit becomes on. This conventional device, however, has a defect in that there is required a rotation waiting time until the relative speed of the track with respect to the laser beam becomes equal to or less than a predetermined speed and thus it takes time to access information. Incidentally, an access operation of the second conventional access control device is similar to that of the first conventional access control device as described above by referring to FIG. 1(b). However, in case of the second conventional access control device, the track pull-in operation is effected in step S5 by repeatedly checking whether the relative speed of the track with respect to the laser beam becomes equal to or less than the predetermined speed, and waiting until the relative speed of the track becomes equal to or less than the predetermined speed.
Turning to FIG. 4, there is shown a third example of the conventional access control device (hereunder sometimes referred to as a third conventional access control device). The configuration of the third conventional access control device is similar to that of the second conventional access control device except the following respects. Namely, a reproduced signal is detected from a plurality of output signals of the photodiode 23 and is then supplied to a demodulator 25. Further, a track No. represented by the output signal of the photodiode 23 is read and transmitted through the demodulator 25 to the MPU 12. Moreover, an output signal LC of the position sensor 6 is inputted to a lens holding circuit 7A for fixedly holding the objective lens 3L at a predetermined position at the time of effecting the seeking operation. Incidentally, an access operation of the third conventional access control device is similar to that of the second conventional access control device. Therefore, the description of the access operation is omitted for simplicity of description.
However, the optical disk system employing the third conventional access control device has drawbacks in that when the optical disk system is subjected to external vibrations or shocks in step S5 in the direction, in which the seeking of a track is effected, the tracking pull-in operation is not stably effected or the control system for controlling the objective lens 3L comes to slightly oscillate or the objective lens 3L largely vibrates to the maximum limit or no focussing signal is obtained and thus the laser beam spot is out of focus, and in that the access time becomes relatively large.
As above stated, the first conventional access control device or system has a drawback in that the tracking pull-in operation cannot be stably effected when the tracking control circuit is changed from the OFF-state thereof to the ON-state thereof after the seeking operation.
Further, the second conventional access control device or system, in which the tracking is performed when the relative speed of a track with respect to a laser beam becomes equal to or less than a predetermined value, has a drawback in that it takes time to access information stored in the optical disk (namely, an access time is relatively long).
Moreover, the third conventional access control device or system has a drawback in that if the optical disk system is subjected to external vibrations or shocks when the tracking control operation is started (namely, the track pull-in operation is commenced) after the seeking operation, the track pull-in operation is not stably performed and in that the access time is relatively long.
The present invention is created to eliminate the drawbacks of the conventional access control devices.
It is, accordingly, an object of the present invention to provide an access control device of an optical disk system provided with an optical disk (like an unused region of a WO CD-ROM) issuing no RF signal (or a signal having a phase shifted 90 degrees from that of a tracking error signal), which can increase stability in performing in the tracking pull-in operation and can reduce the access time.